Some kinds of personal computers and television sets use liquid crystal display devices for displaying still images and moving images. FIG. 14 shows a structure of a conventional color liquid crystal display device. In FIG. 14, 1001 indicates one pixel formed of three, i.e., R, B, and G, pixels. 1002 indicates a liquid crystal display portion including a large number of pixels arranged in rows and columns. 1003 indicates a vertical scanning circuit including a shift register circuit 1004 and a buffer circuit 1005, and can select one row in the liquid crystal display portion. 1006 indicates a horizontal scanning circuit including a shift register circuit 1007, a buffer circuit 1008, and a switch 1009 for applying a signal to one column in the liquid crystal display portion. 1010 and 1001 indicate a vertical scanning line and a signal line, which connect vertical scanning circuit 1003 and horizontal scanning circuit 1006 to each pixel, respectively. FIG. 15 is a circuit diagram showing one pixel of FIG. 14. In FIG. 15, 1101 indicates a thin film transfer (TFT), 1102 indicates a liquid crystal display element, and 1103 indicates a capacitor.
An operation will now be described. When a positive voltage is applied to vertical scanning line 1010, TFT 1101 is turned on to connect signal line 1011 to liquid crystal display element 1102 and capacitor 1103. Thereby, liquid crystal display element 1102 and capacitor 1103 are charged to a potential equal to that on signal line 1011. In a case of a so-called dot sequential drive, horizontal scanning circuit 1006 successively charges the respective column pixels in one row. After all the column pixels are scanned, vertical scanning circuit 1003 sets the voltage on vertical scanning line 1010 to zero or negative value so that TFT 1101 is turned off, and the voltages on liquid crystal display element 1102 and capacitor 1103 are maintained. In a similar manner, the subsequent rows are successively scanned. After vertical scanning circuit 1003 scanned all the rows (which will be referred to as “one frame”), the positive voltage is applied to vertical scanning line 1010 again, and the voltage is written from the signal line to liquid crystal display element 1102 and capacitor 1103. In this manner, all the pixels are successively written a frame at a time, and the display is performed.
Since the liquid crystal display device has the structure described above, it is necessary to maintain a voltage by the electrostatic capacitances of the liquid crystal display element and capacitor for a period (i.e., one frame period) from writing of the signal in one pixel to the next rewriting. However, the voltage lowers due to a finite resistivity of the liquid crystal, leakage in TFT and others, resulting in lowering of display quality, such as flicker. FIGS. 16(a) and 16(b) shows this state. FIG. 16(a) represents an operation with a usual frame frequency of 60 Hz. In this operation, one pixel is rewritten once for each frame period of 1/60 seconds. Therefore, the voltage lowering is small, and the reflectance (brightness) of the pixel does not change so that lowering of the display quality, such as flicker, and lowering of contrast does not occur.
A major portion of the power consumed in the liquid crystal display device is consumed by the powers consumed by the shift register circuits, which perform fast operations in vertical scanning circuit 1003, operating at a frequency of ((frame frequency)×(number of vertical scanning lines)) as well as horizontal scanning circuit 1006 operating at a frequency of ((frame frequency)×(number of vertical scanning lines)×(number of horizontal scanning lines)). For reducing the power consumption, it is effective to reduce the operation frequencies of these circuits or to operate them intermittently. FIG. 16(b) represents a case where the operation frequencies of the horizontal and vertical scanning circuits are lowered for reducing the power consumption. In this case, the rewriting time period (i.e., frame period) of the liquid crystal display element increase, and the voltage declines to an extremely large extent during such a long period. When the display is performed under such conditions, the voltage changes with time so that the reflectance (brightness) changes to a large extent, causing flicker. Also, the average voltage declines so that sufficient contrast cannot be achieved, resulting in low display quality.
An object of the invention is to provide a liquid crystal display device overcoming the above disadvantages, and particularly a liquid crystal display device operating with low power consumption without lowering display quality.